It is important for rubbery polymers (elastomers), particularly conjugated diene-based elastomers (including copolymers of conjugated diene(s) with vinylaromatic materials such as, for example, styrene), that are used in various components of tires to have good compatibility with elastomer reinforcing fillers, such as, for example precipitated silica.
For such purpose, conjugated diene-based elastomers have, for example, been functionalized with in-chain functionality (functionality of units contained within elastomer chain) to enhance their compatibility and promote improved interaction with such elastomer reinforcing fillers. Exemplary of such functionalized diene-based elastomers are, for example, styrene/butadiene copolymer elastomers which are in-chain functionalized with a functionalized styrene monomer which is functionalized with at least one of p- and m-(2-pyrrolidinoethyl) styrene (PES) and m-(2-pyrrolidino-1-methylethyl) alpha-methylstyrene (PAMS) groups. Such in-chain functionality of the elastomer is formed by copolymerization of monomers (e.g. 1,3-butadiene and non-functionalized styrene monomers) together with a small amount of pre-functionalized styrene monomer so that the functionalization of the resulting styrene/butadiene copolymer elastomer, derived from the small amount of functionalized styrene monomer, is positioned within the elastomer itself. Examples of such in-chain functionalized elastomers are, for examples, terpolymers of styrene/PES/butadiene and styrene/PAMS/butadiene. For example, see U.S. Patent Application Publication No. 20040044157
European Patent Publication No. EP 0 693 505 relates to a functionalized polymer where its functionalization is provided by a functionalized initiator instead of a functionalized monomer to therefore lead to a single end functionalized polymer rather than an in-chain functionalized polymer which contains a plurality of in-chain functionalization groups which are contained within the polymer itself.
In said U.S. patent application Ser. No. 10/389,131, it was observed that only a very small amount of the functionalized monomer (e.g. PES) could be utilized to not only provide useful plurality of in-chain functional groups for the elastomer but, also that, for a precipitated silica reinforced rubber composition, an amount of silica coupling agent (e.g. a bis(3-triethoxysilylpropyl) polysulfide) could be significantly reduced.
However, it was also observed that the resultant associated bound functionalized conjugated diene-based elastomer content to the precipitated silica reinforcement may be relatively low when using an in-chain PES functionalized conjugated diene-based elastomer (e.g. a styrene/PES/butadiene terpolymer) when used with a reduced amount of a relatively conventional bis(3-triethoxysilylpropyl) polysulfide silica coupling agent, thus tending to limit the desirability and advantage of utilization of such in-chain functionalization (e.g. with a PES or PAMS derived in-chain functionality) of a conjugated diene-based elastomer for precipitated silica reinforcement of conjugated diene-based elastomers.
Such phenomena may perhaps be due to inadequate chemical bonding between the PES or PAMS (contained in the elastomer chain) and the precipitated silica reinforcement.
In practice, it is considered herein that a relatively high bound elastomer content for the precipitated silica reinforcement in a conjugated diene-based elastomer composition is normally desirable to achieve suitable physical properties for the associated rubber composition for a tire tread such as for example better abrasion resistance.
It therefore remains desirable to increase the bound elastomer content of such in-chain functionalized conjugated diene-based elastomers (e.g. styrene/PES/butadiene terpolymer or styrene/PAMS/butadiene terpolymer elastomer).
For this invention, by slightly coupling an in-chain functionalized styrene/butadiene copolymer elastomer (e.g. styrene/PES/butadiene terpolymer or styrene/PAMS/butadiene terpolymer) at the end of the elastomer chain via an alkoxysilane, particularly an organoalkoxysilane polysulfide, the silica-bound rubber content of a precipitated silica reinforced elastomer composition can be significantly increased.
In the description of this invention, rubber compound, sulfur-cured rubber compound, rubber composition, rubber blend and compounded rubber terms may be used interchangeably to refer to rubber (e.g. elastomer) which has been mixed with rubber compounding ingredients, unless otherwise indicated. The terms “rubber”, “elastomer” and “rubbery polymer” may be used interchangeably unless otherwise indicated. The term “phr” refers to parts by weight of an ingredient per 100 parts by weight of rubber in a rubber composition. Such terms are well known to those having skill in such art.